this morning, after days of silence, the nra, the nation's most powerful gun lobby, announced that it will hold a press conference on friday. and they say they're prepared to make meaningful contributions so tragedies like sandy hook never happen again. are we in the precipice of a sea change? about how this nation deals with guns? >> why the hell can you buy a weapon of mass destruction? that's what it is. that's the first thing that should be gone. >> there have been 70, 70 episodes of school shootings in the united states, since 1994. good lord. really? i mean, should there be that many? >> reporter: big-city mayors and one of the nation's biggest police unions have called for a ban on assault rifles and high-capacity clips. and for some longtime allies of the gun lobby, this week was a tipping point. >> never in my life did i ever think we would have 20 children slaughtered. so, it changed everything. >> reporter: there's signs this morning that everyone, including those that sell guns, are reassessing. at least for now. walmart has stopped online sales of the bushmaster rifle use